1. Field of the Invention
The present invention relates to a light emitting display that controls a light emitting operation of light emitting devices positioned in a matrix form to make a display, more particularly, to a light emitting display that has light emitting devices, such as organic light-emitting diode devices of a structure in which thin films having a thickness on the order of a light wavelength or below are laminated, or the like.
2. Description of the Related Art
An organic light-emitting diode device converts electric energy to optical energy to emit light by injecting holes and electrons into an emissive layer made up of an organic thin film. The light-emitting type display having the organic light-emitting diode device as a light emitting device (referred to as an “OLED display” hereinafter) differs from a non-emissive type display represented by a liquid crystal display in that it is thin and lightweight since it is a self-emissive type, requiring no light source such as back light or the like. Furthermore, the OLED display is characterized in that it has a wide viewing angle and is quick in response.
It is known that an organic light-emitting diode 70 includes: a transparent electrode 200 that is formed on a transparent substrate 6 and functions as an anode; a reflective electrode 300 that has a metal which functions as a cathode; and an organic thin film 100 of a three-layer structure having an electron transporting layer 101, an emissive layer 102 and a hole transporting layer 103, which are in turn stacked between the foregoing electrodes from the cathode side, as shown in FIG. 16. The thickness of the films that constitute the organic light-emitting diode device 70 is, generally, on the order of a wavelength of light, from tens to hundreds of nm or below, and the reflective electrode is specular. Therefore, the light emitted from the emissive layer is influenced by interference. There is a problem with the organic light-emitting diode device 70 that light 2000 which actually travels toward an viewer 1000 changes in emission spectrum and then in color due to a viewing angle. An inorganic electroluminescent device also experiences a similar problem.
A light emitting device that is structured to scatter light to cope with the above problem is disclosed in JP-A-11-329742 and JP-A-2002-270365. These documents describe that color variations relative to a viewing angle caused by interference is significantly reduced because the light emitted from the device is scattered by a light scatterer, and the light traveling in various directions and of different phases are mixed.
A device that significantly reduces color variations relative to a viewing angle which is caused by light interference is also disclosed in JP-A-4-328295. It is structured such that the film of an electron transporting layer is so thick that it includes the secondary maximum value of a film thickness luminance attenuation characteristic, and its amplitude generates luminance exceeding its converging luminance value. For the device, attention is focused on the interference occurring due to the phase difference between the light traveling directly to the viewer and that traveling to the viewer after being reflected on an electrode of the back side amount of the light emitted from the emissive layer, and a condition is set based on the film thickness luminance attenuation characteristic of the electron transporting layer.
An organic light-emitting diode device that utilizes the interference effect is also disclosed in Japanese Patent No. 2846571, wherein an interface between a transparent electrode and a substrate, that between the transparent electrode and a foundation layer of high refractivity, or that between the transparent electrode and a foundation layer of low refractivity is dealt as reflectivity properties and color purity of emission color is improved by controlling of the optical thickness from the anode to cathode.
Typically, a drive type of the OLED display includes an active matrix drive that has a switching element such as a thin film transistor (also referred to as “TFT” hereinafter), and a passive matrix drive in which an electrode constituting an organic light-emitting diode device is directly connected to a scanning line and a data line for driving.
A typical pixel drive circuit of the active matrix drive OLED display has two TFTs of a switching transistor and a driving transistor, and a storage capacitor. The emission of the organic light-emitting diode device is controlled by the pixel drive circuit. Pixels are disposed in each portion of intersections in which n number of data lines supplying data signals (or also referred to as “image signals”) and m number of scanning lines (also referred to as “gate lines” hereinafter) supplying scanning signals are disposed in a matrix of m number of rows by n number of columns.
A pixel is driven by supplying in turn a turn-on voltage from a first (first line) gate line, and by supplying in turn scanning signals to the m number of rows of gate lines within one frame period. In this driving method, while the turn-on voltage is being supplied to a certain gate line, the switching transistors connected to the data lines are all brought into continuity, and a data voltage is supplied to the n number of columns of data lines in synchronization therewith. This is commonly used in an active matrix drive liquid display.
The data voltage is stored in a storage capacitor while the turn-on voltage is being supplied to the gate lines, and is substantially kept during one frame period. The voltage value of the storage capacitor defines the gate voltage of the driving transistor, thereby controlling a value of the current passing through the driving transistor and the light emission of the organic light-emitting diode device. In other words, the active matrix drive OLED display can execute a predetermined light emission during the one frame period.
Compared with the active matrix drive OLED display, in the passive matrix drive OLED display, a current flows to an organic light-emitting diode device with light being emitted only during the time when a certain scanning line is selected. Therefore, in order to acquire the same brightness as that acquired when light is emitted during the entire one frame period in the active matrix drive OLED display, emission brightness of almost several times the scanning line is needed. For this implementation, a voltage as well as a current for driving the organic light-emitting diode device must be increased. As a result, the energy is lost due to heat generation or the like, and thereby power efficiency is reduced.
In this way, the active matrix drive has an advantage over the passive matrix drive in terms of reduction in power consumption.
In the case of implementing the active matrix drive OLED display, a switching device of TFT or the like is required. The switching device is required to electrically drive the organic light-emitting diode device. Taking account of deterioration in specification due to high mobility and a shift in threshold voltage, a polysilicon TFT should preferably be used.
FIG. 17 is a schematic cross-sectional view showing a conventional
OLED display near a pixel having a low temperature polysilicon TFT as a switching device 10. When a low temperature polysilicon TFT is formed on a transparent substrate 6 having a less expensive glass substrate instead of an expensive substrate such like quartz glass substrate, a first foundation layer 11 having SiN for blocking ions and a second foundation layer 102 having SiO are stacked on the transparent substrate 6 so as to be prevented from a problem such as a variation in a threshold voltage due to the mixing in of ions, such as Na, K or the like. Furthermore, a gate insulating layer 16 constituting TFT and additional interlayer insulating layers 18, 20 are stacked on the transparent substrate 6.
As described above, in the active matrix drive OLED display, there exist a plurality of films of different refractive index between the organic light-emitting diode device 70, which includes: organic films 100 having an emissive layer; a transparent electrode 200; and a reflective electrode 300 and the transparent substrate 6. The thickness of these films, which ranges from tens to hundreds of nm, has the influence of interference on the light 2000 which is emitted from the emissive layer and travels to the viewer 1000.
FIG. 18 shows an exemplary measurement result of an emission spectrum of a conventional active matrix drive OLED display. A viewing angle dependency of green emission spectrum is shown in the figure. When the viewing angle changes, a ratio of emission intensity relative to a wavelength changes affected by optical interference: FIG. 19 also illustrates an exemplary measurement of a viewing angle dependency of chromaticity of the conventional active matrix drive OLED display. In the figure, a chromaticity from 0 to 75 degrees of the viewing angle is plotted in every 15 degrees for displaying the three primary colors of red, green and blue, and white. As described above, in the conventional active matrix drive OLED display, there arise unacceptable color variations depending on the viewing angle.
In contrast to this problem, it has conventionally proposed to provide the light-emitting device with a means for scattering light to suppress color variations due to the viewing angle. In this case, when the scattering means, such as a light scattering layers is provided that can sufficiently restrict the affection of interference, light incident on the display from outside scatter-reflects off the scattering means and so can not make a display of black well. The light appears whitish under light circumstances. Thus, it is impossible to get sufficient contrast ratio under light circumstances. This is another problem.
FIG. 20 is a diagram showing a result of a calculation of a viewing angle dependency of interference intensity in a green pixel of the conventional active matrix drive OLED display. As the figure shows, in the conventional active matrix driving OLED display, a number of maximum and minimum values of the interference intensity exist caused by reflection on the interface of the foundation layer and interlayer insulating layer in a visible wavelength range, and a spacing between a wavelength having intensity of interference reaching the maximum value and a wavelength having intensity of interference reaching the minimum value becomes narrower, such as about tens of nm on the side of a short wavelength. Furthermore, the maximum and minimum values move to the side of the short wavelength by about 70 to 140 nm with an increase in the viewing angle.
Therefore, even if a condition is adopted which enhances the intensity of a desired wavelength by controlling the film thickness of the electron transporting layer or an optical thickness from the cathode (reflective electrode) to anode (transparent electrode), on which attention is focused in the conventional technology, the maximum and minimum values, which exist on the long wavelength side rather than the desired wavelength side, move to and appear on an emission wavelength region as the viewing angle increases from 0 to 30, 45, and 60 degrees. Therefore, the emission spectrum observed by the viewer changes, resulting in a change of color.